This application addresses broad Challenge Areas (15): Translational Science and Specific Challenge Topic 15-HL-102: Develop therapeutic strategies for heart, lung, and blood diseases based on microRNA technology. One type of RNA editing involves the conversion of adenosine residues into inosine in double-stranded RNA by the action of ADAR (adenosine deaminase acting on RNA). The A?I RNA editing recodes and diversifies the function of important mammalian genes such as glutamate receptor subunits and serotonin receptor 2C. However, the most common targets for A?I editing are non-coding RNAs that contain inverted repeats of repetitive elements such as Alu and LINE located within introns and 3'UTRs. The biological significance of non-coding, repetitive RNA editing is largely unknown. Recently, we found that primary transcripts of certain miRNA genes (pri-miRNAs) are edited. Editing of pri-miRNAs results in inhibition of their processing or expression of edited mature miRNAs that silence genes different from those targeted by unedited miRNAs. Our findings revealed a previously unknown role for A?I RNA editing in miRNA-mediated gene silencing. The heart is the embryonic tissue where ADAR1 is first detected, at stage E10. ADAR2 expression is detected in the aortic sac at E10.5 and selected regions of embryonic heart including aortic valve at E14.5. ADAR1 null mutant mice have embryonic lethal phenotypes including heart malformation possibly due to hypoproliferation or increased apoptosis of cardiomyocytes. Editing of an unknown target dsRNA(s) by ADAR1 and ADAR2 may play a critical role in the regulation of embryonic heart development. The miRNA-mediated RNA interference recently emerged as a previously unsuspected mechanism to regulate cardiogenesis during development. A select set of miRNAs have been shown to repress genes that regulate proliferation/differentiation of cardiomyocytes during development of embryonic heart. Furthermore, aberrant expression of these miRNAs is associated with congenital ventricular septal defects or pathological conditions of adult heart such as ventricular hypertrophy, hyperplasia and arrhythmias. Our preliminary studies indicate that primary transcripts of miRNA-1, miRNA-133a and miR-208 genes undergo A?I editing. In this Challenge Grant application, we will investigate whether RNA editing of primary transcripts of these miRNAs play a role in the regulation of cardiomyocyte proliferation/differentiation and embryonic heart development. The effects of the RNA editing will be investigated in vitro by a miRNA processing assay and during in vitro induced differentiation of P19CL6 mouse embryonic stem cells to cardiomyocytes. Information obtained in this proposal will be essential for the future development of miRNA-based therapy of various cardiovascular diseases. Public Health Relevance: Our research proposal, based on previously unexplored paradigms, will reveal critical information for better understanding of the mechanisms underlying normal and defective development of heart, and function and disease of adult heart. Information obtained through this research proposal is essential for the future development of a new miRNA-based intervention for prevention of congenital heart defects and pathological cardiac conditions such as cardiac hypertrophy and arrhythmias.